The present invention relates to improved detector and processing circuitry for use with a vorticity measurement optical probe system, such as the system set forth in U.S. Pat. No. 4,385,830, to Webb et al., which is hereby incorporated herein by reference.
In the vorticity measurement system set forth in Webb et al., a plurality of small spherical particles are disposed in the fluid whose vorticity (.omega.) is to be determined. It has been shown that the spherical particles when suspended in a flowing fluid accurately track the local vorticity .omega. of that fluid by rotation with angular velocity .OMEGA.=.omega./2. To be able to observe the rotation, individual optically flat crystalline mirrors are trapped in each particle. The particles are dispersed in a fluid which has substantially the same refractive index as the particles, and the fluid and the particles are illuminated by a light beam. The rotation rates of the reflections from the mirrors are determined by measuring the time delay between signals from a closely spaced pair of detectors. These measurements are accumulated in a data processor such as a microprocessor, and the vorticity spectra and correlations of interest are analyzed on-line by the processor. Various higher correlations are accessible by subsequent calculations and multiple measurement locations along or around a flow channel. Each reflection beam thus provides an accurate measurement of vorticity components.
Although the system set forth in U.S. Pat. No. 4,385,830 works well for flows with large vorticity/velocity ratios such as occur at boundary layers, it has several limitations. These include band width and dynamic range restrictions, limitation to a single vorticity component, (the spanwise component, .omega..sub.z, in the boundary layer), and only one sign of that component. Specifically, in the measurement system set forth in Webb et al., the vorticity of the fluid is determined by measuring the time interval between when the light beam reflected off of the mirror of a rotating sphere strikes a first detector, and when the light beam strikes a second detector. An inherent requirement of the Webb system is that the spheres must be rotating at least at some minimum velocity in order for the reflected beam to move from the first detector to the second detector within the time in which the sphere is in the incident beam's path. Consequently, very low vorticities, as well as zero vorticity can not be measured by the Webb system. Also, the detector circuitry of the Webb system requires that the light beam must strike the first detector, then the second detector in sequence for the time interval to be measured. The system cannot operate in the reverse manner, i.e., light beam striking second detector, then first detector, and for this reason positive and negative signed components of vorticity cannot be measured by the Webb system. Finally, the optical elements of Webb enable only a single component of vorticity, .omega..sub.z, to be measured inasmuch as the detector elements are not two or three dimensional.